GNGTS – Atti del 18° Convegno Nazionale / 01.10
L. Mirabile (1), E. De Marinis (2) and M. Frattini (3)
(1)
Istituto di Oceanologia, Istituto Universitario Navale di Napoli
Free lance scientific consultant
(3) Dipartimento di Geofisica e Vulcanologia, Università Federico II, Napoli
(2)
THE PHLEGREAN FIELDS BENEATH THE SEA THE UNDERWATER
VOLCANIC DISTRICT SOUTHWARD AND WESTWARD
OF NAPLES - ITALY
Following the bradyseimic crises of 1970 and 1984 the lnstitute of Oceanology
of the lstituto Universitario Navale (IUN) of Naples held all around the Pozzuoli gulf
several high resolution reflection seismic surveys, generally single channel,
occasionally multi-channels. In this paper an integrated interpretation of magnetic
(where available) and seismic data is done with the aims to better define the volcanic
area.
The purpose of the present work is to achieve a better knowledge of the
tectonic and subbottom structures of the area and of the location of the top of the
Cenozoic-Mesozoic carbonatic platform and of the overlain Cenozoic sediments or
volcanic filling up, with a special attention steered toward the underwater deposits of
the Campanian Ignimbrite and of the Neapolitan Yellow Tuff.
The tectonic features and the main faults originating the Phlegrean volcanism
was identified and the existence of a basement fault zone, inside the Pozzuoli's Gulf,
1.5 km wide and more than 6 km long, has been documented. An overall uplift of 500
m of the Pozzuoli shoreline was also recognised. Fluid traps were depicted and a
fluid circulation inside the basement (at more than 4000 m depth) is hypothesised to
justify the great amount of vapours and gases in the sedimentary column.
To rationalise the work, the geophysical analysis was carried out splitting the
whole territory of the study into four different sub-areas. Each area has been chosen
with a data coverage as homogenous as possible and with subbottom structure
similarity:
 the Tyrrenian margin, South of the Gulf of Naples, characterised by
submerged volcanoes and the rise of the Dohrn and Magnaghi canyons;
 the Naples’ Gulf, where the sedimentation shows a wide disposal of
Campanian Ignimbrite (CI) and Neapolitan Yellow Tuff (NYT);
 the North-Western Tyrrhenian margin, where magnetic, gravimetric and
seismic data are available and where a good correlation exists between
ashore and offshore structures;
 the Gulf Of Pozzuoli, characterised by the presence of submerged volcanic
banks and by the carbonatic block recognisable between Magnaghi and Dohrn
canyons.
THE THYRRHENIAN BOUNDARY, SOUTH OF THE GULF OF NAPLES
In that area more multichannel seismic and detailed magnetic surveys are
necessary in order to define the tectonic trends and the location of the basement
faults, and to explain the origin and nature of the volcanic intrusions. Here the Ischia
GNGTS – Atti del 18° Convegno Nazionale / 01.10
volcano, with a radius of about 20 km at the sea-bottom, reveals to be the biggest
volcano in the area.
THE NORTH-WESTERN TYRRHENIAN MARGIN
The seismic profile alongshore, in the southern part, suggests the presence of
lava layer at 1.5 s T.W.T. deeping abruptly to more than 3 s WNW of the Procida
Pozzuoli magnetic maximum. The sharpness of the transition to the sediments and
volcano-sediments depression may suggest the presence of faults in the limestone
basement that, however, was not imaged in the seismic sections, neither beneath the
lava layer nor under the softer materials filling the depression, which must be deeper
than 4000 meters. The northern end shows again lava deposits between 2 and 3 s
that can be associated to less recent Villa Literno-Parete volcanism, related to the
tectonic activity of the 41st parallel strike-slip.
WESTERN NAPLES' GULF AND PHLEGREAN FIELDS
The limestone basement emerging in Capri and Sorrento presents northward a
series of structural highs from tilted blocks at increasing absolute depth from South to
North. The first from south is Fuori bank that lies by Magnaghi and Dorhn canyons. It
result displaced by a relative left lateral movement from the corresponding Aloha
bank. The displacement is of approximately 4 km, along a possible NW-SE fault,
linked to the Southern Tyrrhenian tectonic evolution, and nearly parallel to the WNWESE strike slip separating the Southern from the Northern Tyrrhenian domains at the
41.st parallel latitude. The dramatic depression of the Phlegrean Fields area is
confirmed by the seismic markers referred to the top of Miocene unconformity and
top of the carbonate platforms on the north-western margin, north of Procida and on
the Gulf of Naples margin, on the south-eastern margin. The volcanic area
correspond to the higher tectonic deformation along the direction from Ischia,
Procida, Phlegrean Fields.
The Phlegrean volcanism originates from the prolongation of Magnaghi Canyon.
The two canyons bound the carbonatic blocks that are displaced to the left by a NWSE directed sequence of fractures. The Aloha block bounds the Eastern branch of
the Magnaghi fault, near its fork. The Magnaghi fault zone continues inside the Gulf
of Pozzuoli becoming a very important source of volcanic products. The fracture is
1.5 km wide and 6 km long and probably reaches, ashore, the fault passing close to
the Pozzuoli's magnetic maximum, some km northwards from the shoreline. There is
no evidence of any eventual caldera and all the Phlegrean volcanism may be
explained by the presence of crustal faults actually recognised and mapped utilising
the seismic profiling.